1. Field of the Invention
The present invention relates to a transmission terminal that transmits information, a reception terminal that receives that information, and an information distribution system that includes these terminals.
2. Description of the Related Art
In recent years, home wireless networks have become widespread. As wireless network systems that are currently becoming widespread, there are wireless LANs, whose purpose is to connect computer devices such as personal computers to the Internet. Henceforth, as wireless network systems that will newly become widespread in homes, digital information home appliance network systems and sensor network systems are expected.
These networks will cooperate with each other, so that services such as, for example, controlling an air conditioner to conserve its power on the basis of a detailed spatial temperature distribution gathered by a temperature sensor or advising a user through a television receiver of an abnormality in his/her body detected by a biosensor will become possible.
The issue with such home wireless network systems is that a third party can easily illegally acquire personal information that is inside the home and can illegally access devices that are inside the home because home wireless network systems exchange information wirelessly. For example, there is concern that information corresponding to the privacy of an individual that has been detected by a biosensor may be intercepted and concern over harassment such as switching the power of a television receiver ON and OFF from the outside. Thus, it is believed that information encryption and access control (device authentication) will become increasingly important in future home wireless network systems.
Here, a case will be supposed where various devices equipped with a wireless communication function have extremely limited computing capability and memory capacity in comparison to a computer device or the like. In a communication network using such wireless terminal devices, what becomes an issue is how wireless terminal devices that are to be newly added to the network system (wireless terminal devices in which key information for authentication has not been set beforehand) and the network to which those wireless terminal devices are to be added set key information for encryption and authentication securely. As long as the wireless terminal devices and the network can set this initial key information (below, called an “initial key”) securely, they can securely communicate between arbitrary wireless communication terminals and can also perform updating of the key information securely.
As methods of securely setting the initial key, various methods exist, such as, for example, methods where a user manually sets the initial key, methods that utilize a wired connection to deliver the key, and methods that utilize wireless communication. Among these, in methods that utilize wireless communication to perform setting of the initial key, it is necessary to prevent a third party from intercepting the wireless communication and illegally acquiring the key information.
As those methods, for example, a method that uses infrared communication, which has directionality and is difficult to intercept, to deliver the key and a method that delivers the key by non-contact communication that can only be read nearby using an RFID tag (a wireless ID tag) or the like are conceivable. However, these methods require separate interfaces in the wireless terminals when the terminals are not equipped with the above-described infrared or RFID interfaces.
As methods of securely realizing the sharing of an initial key utilizing wireless communication without having to utilize separate interfaces and even if a third party present in the surrounding area were to intercept the wireless communication, techniques based on public key encryption techniques represented by Diffie-Hellman key exchange, for example, are known. However, sometimes these systems become a burden on wireless communication terminals that have limited computing capability. Further, sometimes installing a public key encryption operation algorithm just for sharing this initial key becomes a burden on wireless communication terminals that have limited memory capacity.
In relation to the above issue, as a technology whose purpose is “to reduce the total cost of a system and to lessen the burden on a manager by making it possible to securely perform initial registration of a user only at a fixed base station and to distribute an encryption key for use in communication”, there has been proposed an encryption key distributing method and a wireless network system (see Japanese Patent Publication Laid-open (JP-A) No. 2005-79975) where: “An output-variable configuration base station 101 that includes the function of lowering its wireless output to set a small initial registration area 102 is disposed in a system. When there is a request for initial registration from a wireless terminal 103, this fixed base station 101 lowers its wireless output to set the small initial registration area 102, and, in this state, the fixed base station 101 registers the wireless terminal that made the request for registration and distributes an encryption key to that wireless terminal. Thereafter, the fixed base station 101 returns its wireless output to normal to set a communicable area 105 and performs communication with the wireless terminal.”
In the technology described in JP-A No. 2005-79975, the fixed base station 101 lowers its wireless output to set the small initial registration area 102 and performs key distribution in that area It becomes difficult for an intercepting terminal located in a range far from this initial registration area to reliably intercept the encryption key because the bit error rate becomes high, so the security of key distribution rises.
However, in the technology described in JP-A No. 2005-79975, it is not clear whether or not the intercepting terminal mistakes the received bits.
Moreover, the range in which the fixed base station can lower its wireless output relies on the function of the wireless terminal, so sometimes the fixed base station cannot lower its wireless output to the extent that it can perform key distribution securely.
For that reason, there have been desired a transmission terminal, a reception terminal and an information distribution system that can, even if wireless output reduction is limited, transmit information such that there are more bit errors in an intercepting terminal and improve the degree of security of information transmission.
For this reason, there has been proposed a novel and improved important information transmission system where only wireless terminal devices that can acquire information with a certain probability can continue communication under a restricted communication environment and which system is capable of preventing interception by other unauthorized wireless terminal devices (see JP-A No. 2007-235516). However, further improvement of the security level is demanded with respect to this system also.